First create a new CSS file for formatting–go to file -> new file, CSS File. Then in the YAML tell it about the CSS in the output (needs to be in the same folder), which it’ll use when it renders
If you don’t know what an element is called to google it: CTRL: right clikc + Inspect element -> see info for development (e.g. h3 for Header 3 and if you want to overwrite it you’ll know what it’s called)